Method for inspecting exposure apparatus

ABSTRACT

Light emitted from an illumination optical system is guided to a photomask where a pattern is formed of an optical member including a light transmission pattern as a diffraction grating pattern, in which a light transmission part and a opaque part are repeated in a finite period and a periphery of the light transmission pattern is shielded by a opaque area, such that a plurality of ratios are given between the light transmission part and the opaque part. Diffraction light, which has passed through the photomask, is irradiated on a projection optical system, thereby to transfer a pattern reflecting an intensity distribution of the diffraction light to a wafer. A change of transmittance depending on a light path of the projection optical system is measured, based on a pattern image of the diffraction light transferred to the wafer. Pattern transfer is carried out in a non-conjugate state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2000-036690, filed Feb. 15,2000, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for inspecting a projectionexposure apparatus used for manufacturing a semiconductor device, andparticularly to a method for inspecting performance of a projectionoptical system of an exposure apparatus.

2. Description of the Related Art

A lithography technique is generally used to manufacture a circuitpattern of a semiconductor device. In a projection exposure apparatusused in a lithography process, light emitted from an illuminationoptical system enters a photomask on which a circuit pattern is drawn.Light passing through the photomask is converged by a projection opticalsystem. Further, in general cases, the circuit pattern of the photomaskis focused and projected on a substrate applied with a photosensitivematerial, e.g., a silicon wafer applied with photoresist.

Recently, as the semiconductor device pattern to be formed is downsized,the dimension of the pattern to be formed by the optical lithographybecomes severer.

In case of the exposure apparatus, as the pattern of the semiconductordevice in comparison with the exposure wavelength is shrunk more,diffraction of light becomes more remarkable. Also, it is known that thediffraction angle increases as the period of the pattern decreases. Toform a micro pattern, the diffraction light propagating in the directionin which it goes away from the optical axis needs to be captured andconverged onto the wafer. Therefore, the diameter of the projectionoptical system needs to be increased in order to form a more microscopicpattern. In other words, the numerical aperture NA of a projectionoptical system needs to be increased. In case of the exposure using aphotomask which has a one-dimensional periodic pattern such as line &space pattern, a plurality of discrete diffraction light are occurred.The discrete diffraction light are straight zeroth-order diffractionlight, first-order up to higher-order diffraction light which havepredetermined diffraction angle. In order to form a one-dimensionalperiodic pattern on the wafer, first-order diffraction light needs to becaptured and converged with zeroth-order diffraction light.

Meanwhile, if the projection lens forming part of the projection opticalsystem becomes large, a problem occurs in that light transmittancedepending on the light path changes. In case of exposing a relativelylarge pattern with respect to the exposure wavelength, the lightdiffraction angle is small. In this case, only the portion of light thatpasses through the optical axis of the projection lens contributes tofocusing of an image. That is, the paths of zeroth-order diffractionlight and first-order diffraction light that are used for focusing animage are different from each other. Accordingly, the intensity of eachdiffraction light is not influenced by changes of the transmittance ofthe projection optical system.

In contrast, the diffraction angle is large in case of exposing a micropattern, and therefore, zeroth-order diffraction light and first-orderdiffraction light are different from each other. Accordingly, if thetransmittance in the projection optical system changes depending onlight paths, diffraction light which reaches a wafer is influenced bychanges of the transmittance, and as a result, the intensity of eachdiffraction light changes.

In conjunction of design of the projection optical system, changes ofthe transmittance depending on the light paths are not caused. But inpractice, the drawbacks can be occurred imperfect anti-reflectioncoating on lens surface, light absorption of lens material, and thelike. However, proposals have not yet been made for a method of directlymeasuring this phenomenon without disassembling the exposure apparatus.

A transmittance change depending on the light paths causes theintensities of zeroth-order diffraction light and first-orderdiffraction light to change. Since photoresist pattern on a wafer isformed by interference between these diffraction lights, a change of theintensities influences the pattern image focusing performance. As aresult of this, it is considered that the micro pattern transferringperformance of the projection optical system is deteriorated.

If a micro periodic pattern is formed by interference betweenzeroth-order diffraction light and first-order diffraction light, lightgenerated by the interference constructs a bright part and a dark part.The degree of brightness is expressed as an amount of contrast. Ifbright and dark parts are clearly distinguished from each other, it iscalled “high contrast”. The higher the contrast of interference light,the easier the transfer of the pattern onto the wafer. In other words,the contrast should desirably be high in order to widen the focus marginand the exposure dose margin. The contrast is determined by amplitudeand phases of lights which interference each other.

If a circuit pattern is designed supposing that drawbacks describedabove do not occur, the contrast of interference light formed on thewafer is rendered insufficiently high. As a result, no pattern may beformed. At present, shrinkage of patterns has progressed and lithographydesign using simulations has come to have a significant meaning. It isundesirable that unexpected drawbacks of this kind occur in the exposureapparatus. In the process of assembling an exposure apparatus, drawbacksshould be removed or extents of drawbacks should previously measured andwhich then have to be taken into consideration in case of estimate anddesigning of to-be-formed pattern based on exposure simulations.

An example of measurement of contrast, which has been conventionallycarried out, will be explained with reference to FIG. 1. FIG. 1 showsrelationship between formed photoresist patterns (left side) andrelative light intensities I (=1/D) (right side). The contrast isexpressed by the following expression with use of a light intensity I1at peaks of light intensity and a value I5 at a minimum light intensitybetween peaks. $\begin{matrix}{({contrast}) = {\left( {{I1} - {I5}} \right)/\left( {{I1} + {I5}} \right)}} \\{= {\left( {{1/{D1}} - {1/{D5}}} \right)/\left( {{1/{D1}} + {1/{D5}}} \right)}} \\{= {\left( {{D5} - {D1}} \right)/\left( {{D5} + {D1}} \right)}}\end{matrix}$

In the expression, the intensity I5 at which the light intensity comesto peaks is an intensity of the minimum between peaks of lightintensities. Although presence or absence of reduction of the contrastcan be confirmed by the method shown in FIG. 1, factors which causereduction of the contrast is very difficult to specify.

Another phenomenon which is caused by a change of the diffractionintensity is a positional shift of pattern depending on focusing onto awafer, which is caused by the gravity center of the intensity of thediffraction light shifts from the center of the projection opticalsystem. Where a line-and-space pattern are cited as an example, two ofpositive and negative first-order diffraction lights are generated witha center of zeroth-order diffraction light taken as a symmetry point. Ifthere is a difference between intensities of the positive and negativefirst-order diffraction lights, the position where the pattern is formedshifts depending on the defocus amount of the wafer.

The shift of the position where the pattern is formed depending on thedefocus amount of the wafer is occurred due to factors other than atransmittance change depending on the light paths, such as comaaberration or illumination telecentricity error. Therefore, it isdifficult to specify the factor which causes the shift of the positiononly by the measurement of the relationship between the defocus andmisalignment of the pattern.

BRIEF SUMMARY OF THE INVENTION

The present invention has an object of providing a method for inspectingan exposure apparatus capable of specifying a change of the lighttransmittance depending on the light path.

The present invention provides a method for inspecting an exposureapparatus, comprising: a step of guiding light emitted from anillumination optical system to a photomask where a pattern is formed ofan optical member including a light transmission pattern as adiffraction grating pattern, in which a light transmission part and aopaque part are repeated in a predetermined direction, a plurality ofratios are given between a length of the light transmission part and alength of the opaque part in a repetition direction, and a periphery ofthe light transmission pattern is shielded by a opaque area, such that aplurality of ratios are given between the light transmission part andthe opaque part;

a step of irradiating diffraction light, which has passed through thephotomask, on a projection optical system, thereby to transfer a patternreflecting an intensity distribution of the diffraction light to awafer; and

a step of measuring a change of transmittance depending on a light pathof the projection optical system, based on a pattern image of thediffraction light transferred to the wafer.

When inspecting a projection optical system of an exposure apparatus inthe present invention, light emitted from a light source is guided to aphotomask and light passing through the photomask is irradiated on likenormal pattern exposure, thereby to transfer a pattern image reflectingan intensity distribution of the diffraction light on a wafer.

When transferring a pattern in the present invention, a lighttransmission pattern in which light transmission parts and opaque partsare repeated at a finite period is formed on a photomask, and therefore,diffraction light is obtained.

In addition, the photomask and the wafer are rendered non-conjugate withrespect to the projection optical system. In this manner, patterntransfer can be performed in a state in which diffraction lights ofzeroth-order up to higher-order are separated from each other anddiffraction light components have sufficient sizes. In the presentinvention, where NA is a numerical aperture of the projection opticalsystem in a wafer side, λ is a exposure length, σ is a coherence factor,and M is a magnification of the photomask, the period of the diffractiongrating pattern is set so as to satisfy p>Mλ/NA(1+σ). In this manner,first-order diffraction light can be transferred to the wafer withoutbeing shaded by the aperture stop, so that the light intensitydistribution can be inspected based on the transferred pattern image.

By observing patterns thus obtained on a wafer, it is possible tomeasure changes of light transmittance depending on light paths.

Specifically, by taking exposure using light transmission patterns whichhave the light transmission parts and the opaque parts the ratio ofwhich (the ratio between the light transmission parts and the opaqueparts) are different from each other, a plurality of resist patterns aretransferred onto the wafer. By analyzing the resist patterns, anequal-intensity contour plot of light intensity distributions isobtained. It is thus possible to measure the light transmittancedepending on the paths of the projection optical system based on thecontour plot of light intensity distributions.

More desirably, the photomask where the diffraction patterns are formedis constructed as a attenuated phase shift mask. Namely, the diffractionpattern is constructed by a light transmission part and asemi-transparent phase shift part at which the phase is shifted from thelight transmission part. In this respect, the duty ratio of thediffraction grating can be adjusted so that the intensity ofzeroth-order diffraction light can be approximately zero. In this case,patterns depending on the zeroth-order diffraction light are nottransferred. Therefore, only the first-order diffraction light can beobserved so that first-order diffraction light components close to thelight axis of the optical system of the exposure apparatus can beobserved.

In addition, the non-conjugate state in which the photomask and thewafer are non-conjugate with respect to the projection optical system isrealized by arranging the opaque part of the light optical member on asurface opposite to a surface where the optical member of the photomaskused for device pattern exposure is arranged. That is, the photomask isattached to a mask stage, reversed from the state in case of devicepattern exposure. In this manner, a non-conjugate state can be createdvery simply while maintaining the structure of the exposure apparatusused for pattern exposure. Of course, at least one of the photomask andthe wafer may be shifted from a conjugate position in the light axisdirection.

Also, conditions are set so as to satisfy a relationship of0.4(ndλ)^(1/2)≦r≦(ndλ)^(1/2) where the light transmission pattern has acircular shape having a radius r, d is thickness of the photomask, λ isan exposure wavelength, and n is a refractive index of material of thephotomask at the exposure wavelength λ. In this manner, the resolutionof the transferred pattern image can be improved and the resist patternswhich are suitable for measurement can be obtained.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 are an explanatory view for a conventional contrast measurementmethod;

FIG. 2 is a view showing the entire structure of an exposure apparatusas a target according to the first embodiment of the present invention;

FIGS. 3A and 3B are views showing the entire structure of a photomask 3incorporated in the exposure apparatus according to the embodiment;

FIG. 4 is a plan view showing photoresist patterns obtained by patternexposure according to the embodiment;

FIG. 5 are plan views showing photoresist patterns obtained by fivetimes of pattern exposure according to five kinds of exposure dosesaccording to the embodiment;

FIGS. 6A, 6B, 6C, 6D, 6E, 6F, 6G, 6H are views showing a modificationexample of the embodiment in which inspections are made in a method ofchanging the duty ratio;

FIGS. 7A, 7B, 7C, 7D, 7E, 7F, 7G, 7H are views showing a modificationexample of the embodiment in which diffraction patterns having aplurality of periods are used to make an inspection;

FIGS. 8A to 8B are views showing a modification example of theembodiment in which inspections are carried out with use of diffractionpatterns which have line lengthwise directions in a plurality ofdirections;

FIGS. 9A, 9B, 9C are views showing the entire structure of a photomaskused in the second embodiment of the present invention;

FIG. 10 is a view schematically showing photoresist pattern obtained bypattern exposure according to the embodiment;

FIG. 11 is a plan view showing photoresist patterns obtained by sixtimes of exposure depending on six kinds of exposure doses, accordingthe embodiment;

FIGS. 12A and 12B are plan views showing main parts of a photomask usedin the third embodiment of the present invention;

FIG. 13 is a view schematically showing photoresist patterns obtained bypattern exposure according to the embodiment;

FIG. 14 is a plan view showing a photomask used for inspecting anexposure apparatus according to the third embodiment of the presentinvention;

FIGS. 15A and 15B are views showing shapes of photoresist patternsformed by an inspection method according to the embodiment, comparedwith another embodiment;

FIG. 16 is a schematic view showing light paths of first-orderdiffraction light in an inspection according to the embodiment;

FIGS. 17A and 17B are plan views showing modification examples ofpinhole patterns formed on photomasks of the present invention;

FIGS. 18A, 18B, 18C are plan views showing modification examples ofphotomasks of the present invention;

FIG. 19 is a plan view showing modification examples of photomasks ofthe present invention;

FIG. 20 is a view schematically showing photoresist patterns obtained bythe pattern in the FIG. 19;

FIG. 21 is a graph showing a sensitivity curve; and

FIG. 22 is a graph showing a sensitivity curve.

DETAILED DESCRIPTION OF THE INVENTION

In the following, embodiments of the present invention will be explainedwith reference to the drawings.

(First Embodiment)

FIG. 2 is a view showing the entire structure of an exposure apparatusas an inspection target according to the first embodiment of the presentinvention. In the present embodiment, explanation will be made of a caseof an example in which a KrF excimer laser reduction projection exposureapparatus (λ: 248 nm, NA: 0.6, σ: 0.3, mask magnification M: 4) isinspected.

FIG. 2 is a view showing the entire structure of a reduction projectionexposure apparatus as an inspection target of the present embodiment. Asshown in FIG. 2, a light source 1, a photomask 3, a projection opticalsystem 4, and a wafer 5 are arranged in this order along the light pathof exposure light 2.

FIGS. 3A and 3B are views showing the photomask 3 incorporated in theexposure apparatus. As shown in the plan view of the entire structure inFIG. 3A, a pinhole pattern 22 is arranged as a light transmissionpattern in the photomask 3. Further, the periphery of this pinholepattern 22 serves as a opaque area 23. The diameter of the pinholepattern 22 as dimensions on a photomask is 80 μm.

FIG. 3B is an enlarged view of a part of the pinhole pattern 22 of thephotomask 3. As shown in FIG. 3B, a diffraction pattern 24 constructedby line and space patterns is formed on the pinhole pattern 22 of thephotomask 3. This diffraction pattern 24 includes opaque parts 25 andlight transmission parts 26. The period is 1.6 μm and the ratio betweenthe width of each shielding part 25 and the width of each lighttransmission part 26 is 1:1.

In the present embodiment, the period of the diffraction gratingssatisfies p>Mλ/NA(1+σ), where the aperture of the projection opticalsystem 4 in the side of the wafer 5 is NA and the coherence factor ofthe exposure apparatus is σ, the exposure wavelength is λ, and themagnification of the mask is M. As a result of this, zeroth-orderdiffraction light and first-order diffraction light can be transferredto the wafer 5, separated from each other. Accordingly, the lightintensity distribution can be inspected without interference withzeroth-order diffraction light.

The photomask 3 described above is provided such that the surface wherethe pattern is formed is arranged in the side of the illuminationoptical system 1 along the light path formed from the illuminationoptical system to the wafer 5, as shown in FIG. 2, and pattern exposureis carried out. Conventionally, in case of device pattern exposure, thephotomask 3 is set such that the actual pattern forming surface is setin the side of the wafer 5 along the light path formed from theillumination optical system to the wafer 5. By attaching the photomask 3to a mask stage with the mask reversed from the arrangement in actualpattern exposure as in the present embodiment, the wafer 5 and thephotomask 3 can be rendered non-conjugate with respect to the projectionoptical system 4.

FIG. 4 is a plan view schematically showing photoresist pattern formedon the wafer 5 by pattern exposure. As shown in FIG. 4, the light reacharea 31 is an area to which light can reach from the illuminationoptical system 1 through the projection optical system 4. This isbecause the light which passes through the projection optical system 4by a pupil 4 forming part of the projection optical system 4 isconverged, and a boundary line which defines the light reach area 31corresponds to the outer periphery of the pupil 4 a.

As shown in FIG. 2, zeroth-order diffraction light 6, positivefirst-order diffraction light 7 a, and negative first-order diffractionlight 7 b pass through the inside of the pupil 4 a. Accordingly, azeroth-order diffraction pattern 32 and positive and negativefirst-order diffraction light patterns 33 a and 33 b are created on thewafer 5. These diffraction light patterns 32, 33 a, and 33 b have shapessimilar to the cross-sectional shape of the light emitted from theillumination optical system 1 of the exposure apparatus. In addition,these patterns have sizes which reflect the value of the coherencefactor σ expressing the size of illumination.

In addition, images which reflect the intensities of the diffractionlights 6, 7 a, and 7 b are formed on the wafer 5. The positive andnegative first-order diffraction lights 7 a and 7 b are partiallyshielded by the aperture stop at the pupil plane 4 a of the projectionoptical system 4 and their shapes are chipped thereby.

This kind of photoresist pattern is observed, for example, by an opticalmicroscope, and a boundary between an area where photoresist wasstripped and an area where photoresist was remained is defined.Specifically, the boundaries between the diffraction patterns 32, 33 a,33 b, and the other areas are the boundaries defined here.

Pattern exposure explained above is carried out for a plurality of timeswith the exposure dose changed. In the present embodiment, for example,exposure is carried out five times. FIG. 5 shows plan views ofphotoresist patterns obtained through five times of exposure. References41 to 45 respectively show plan views of photoresist patterns atexposure doses. The photoresist pattern 41 shows the case where theexposure dose is the lowest from which the exposure dose increases inthe order of 42, 43, . . . . The photoresist pattern 45 shows the casewhere the exposure dose is the highest.

As shown in FIG. 5, it is known that the sizes of the diffraction lightpatterns 32, 33 a, and 33 b vary by changing the exposure dose. Primarydiffraction light patterns 33 a and 33 b which cannot substantiallyappeared at a low exposure amount can appear more clearly as theexposure dose is increased. If the exposure dose thus varies, theboundaries between the diffraction patterns 32, 33 a, and 33 b varyaccordingly.

Based of the five kinds of photoresist patterns 41 to 45 obtained bythus changing the exposure dose, a difference is obtained between thepositive and negative first-order diffraction lights. Specifically, ashape of a diffraction light pattern as a reference (which will behereinafter called a reference pattern) is determined. Further, patternsare selected each of which is most similar to this reference patternfrom the five kinds of photoresist patterns, thereby to measure theexposure dose of the selected pattern. For example, the exposure dosewhen the positive first-order diffraction light pattern 33 a becomes areference is expressed as Ma, and the exposure dose when the negativediffraction light pattern 33 b becomes a reference is expressed as Mb.

Further, the light intensity is calculated from the obtained exposuredoses Ma and Mb based on the inverse proportional relationship betweenthe light intensities and the exposure doses. Therefore, 1/Ma:1/Mb=Ia:Ibis obtained where Ia is a light intensity at which the positivefirst-order diffraction light pattern 33 a is finished to have areference shape, and Ib is a light intensity at which the negativefirst-order diffraction light pattern 33 b is finished to have areference shape. For example, if Ma:1/Mb=9:10 is given, the ratio of thediffraction light intensities is Ia:Ib=1/9:1/10=10:9.

In case of an ideal projection optical system, positive and negativediffraction lights which has passed through the projection opticalsystem have an equal intensity, and therefore, the diffraction lightintensities Ia and Ib become equal to each other. Meanwhile, if the filmthickness of an anti-reflection film attached to the surface of a lensforming part of the projection optical system is not partially adesirable value because of the non-uniformity of the film thickness ofthe anti-reflection film or the lens itself has light absorptioncharacteristics, the light transmittance is lowered at that part. Thepositive first-order diffraction lights are similar to negativefirst-order diffraction lights right after occurrence, and the positiveand negative first-order diffraction lights have passed through theinside of the projection optical system, so each of the positive andnegative first-order diffraction lights can be passed through thedifferent paths with each own transmittance. As a result, the intensitydiffers between the positive and negative first-order diffractionlights. In the above example, it is apparent that the difference of thetransmittance therebetween is 10:9 where expressed as a ratio. Thisratio indicates a transmittance ratio of the light paths through whichthe positive and negative first-order diffraction lights has passed. Asdescribed above, it is possible to inspect the transmittance of theprojection optical system, based on the exposure doses Ma and Mb.

Thus, according to the present embodiment, the periphery of thediffraction pattern where light transmission parts and opaque parts arerepeated at a finite period is shielded by a opaque area, and thepattern is arranged with the pattern forming surface reversed from thestate of actual pattern exposure, so that light from the illuminationoptical system is let pass through the photomask. In this manner, achange of the light transmittance depending on the path of theprojection optical system can be measured in a state in which thephotomask and the wafer are not conjugate with respect to the projectionoptical system.

The present invention is not limited to the embodiment described above.Although a pattern having a light transmission part and a opaque part,the pattern width of which is at a ratio of 1:1, is used as the line andspace pattern, the present invention is not limited thereto but theratio may be set to any ratio that can generate positive and negativefirst-order diffraction lights.

Although an inspection has been made with the exposure dose changed, theratio of the light intensity may be measured by another method. Forexample, a plurality of patterns having one same period and one sameshape may be provided, and pattern exposure may be carried out whilechanging the duty ratio of each pattern, i.e., the ratio of the lengthof the light transmission parts to the period of the patterns. In thiscase, the projection optical system can be inspected through oneexposure by respectively forming patterns having different duty ratioson photomasks.

FIGS. 6A to 6H are explanatory views for principles in which exposure ofone time is enough by adopting the method of changing the duty ratio.References 51 a to 51 d in FIGS. 6A to 6D denote mask patterns formed onone photomask. FIGS. 6E to 6H show schematically photoresist patternsobtained respectively from the mask patterns 51 a to 51 d.

Although the mask patterns 51 a to 51 d have a common point that a holepoint pattern 22 with a diameter of 80 μm (as dimensions on a photomask)is formed in each opaque area 23, they have different duty ratios fromeach other. The duty ratio decreases in the order from the pattern 51 a.Therefore, the pattern 51 a has the highest duty ratio and the pattern51 d has the lowest duty ratio.

If a pattern is transferred with use of the patterns 51 a to 51 d havingfour kinds of duty ratios and one same period, the shapes of therespective photoresist patterns become different from each other, asshown in FIGS. 6E to 6H. More specifically, the diffraction lightpatterns 32, 33 a, and 33 b become different from each other. Althoughthe diffraction light patterns 32, 33 a, and 33 b are expressed ascircular shapes in FIG. 4, the patterns have thus circular shapes incase where the light irradiated from the illumination optical system hasa circular irradiation area. An illumination optical system can beconstructed by an aggregation of a plurality of point light sources, asspecifically shown in FIGS. 6E to 6H.

Accordingly, the diffraction light patterns 32, 33 a, and 33 b areconstructed by an aggregation of a plurality of circular patterns. Bychanging the duty ratio, the light intensity of the first-orderdiffraction light which has occurred can be changed. In case of theline-and-space pattern, the relationship between the duty ratio and thelight intensity of the first-order diffraction light is expressed by thefollowing expression:

I=I0 sin²(πx)

where x is the duty ratio, I is the light intensity of the first-orderdiffraction light, and I0 is a proportionality constant. Therefore, thesame information as that obtained in case of exposing one same patternwith a plurality of different exposure doses can be obtained byperforming exposures with use of these patterns 51 a to 51 d. Althoughthis modification shows a case where patterns having different dutyratios are arranged on one photomask, the patterns may naturally beprovided on a plurality of photomasks.

In addition, the period of the diffraction pattern 24 is set to 1.6 μmin the present embodiment. However, the invention is not limitedthereto. A more detailed inspection can be achieved by a plurality ofdiffraction patterns not having one same diffraction pattern having onesingle period as in the present invention but having a plurality ofperiods. A modification example in which an inspection is carried outwith use of diffraction patterns having a plurality of periods will beexplained with reference to FIGS. 7A to 7H.

The references 61 a to 61 d shown in FIGS. 7A to 7D denote mask patternsformed on one photomask. FIGS. 7E to 7H schematically show photoresistpatterns on a wafer, which are respectively obtained by mask patterns 61a to 61 d.

The mask patterns 61 a to 61 d have a common point that a hole pattern22 having a diameter of 80 μm is formed in each opaque area 23. Thesepatterns, however, have periods different from each other. The perioddecreases in the order from the pattern 61 a. Therefore, the pattern 61a has the largest period and the pattern 61 d has the smallest period.The diffraction patterns of the patterns 61 a to 61 d have a common dutyratio of 0.5.

If a pattern is transferred with use of the patterns 61 a to 61 d havingpattern periods of these four kinds and one same duty ratio, transferpositions of positive and negative first-order diffraction lightpatterns 33 a and 33 b as shown in FIGS. 7E to 7H differ. This is basedon the principle that the light paths of the positive and negativefirst-order diffraction lights 7 a and 7 b can be changed by changingthe period of the diffraction grating as described previously.Accordingly, the light paths of the positive and negative first-orderdiffraction lights 7 a and 7 b differ in the projection optical system4. Measurement of the positive and negative first-order diffractionlights 7 a and 7 b that have passed these different paths indicatesmeasurement of light that has passed through the vicinity of the centerof a pupil plane forming part of the projection optical system 4 up tolight that has passed through the vicinity of the periphery of the pupilplane.

These differences between the light paths apply in the same manner tothe photoresist patterns. The component which runs straight afterpassing the photomask 3 is irradiated on the vicinity of the center ofthe light reach area 31, and the component which runs at a largediffraction angle after passing the photomask 3 is irradiated on theperipheral part of the light reach area 31. Accordingly, as shown inFIGS. 6E to 6H, observation of light diffraction patterns 33 a and 33 bformed at a plurality of positions means that information of intensitiesof lights in the radial direction of the projection optical system.Thus, the light transmittance distribution of the light path withrespect to the radial direction of the projection optical system can bemeasured by thus using a plurality of periodic patterns.

The periods of the diffraction patterns may be provided so as to coverall the diameters in the radial directions of the circular areaindicated by the light reach area 31. In this manner, the distributionof the light transmittance of the lens forming part of the projectionoptical system can be inspected with respect to all positions on onediameter. In addition, patterns having these plural periods may beprovided on one photomask. In this manner, a large amount of informationcan be obtained by one time of exposure, and the inspection time isshortened. As a result, measurement becomes simple and easy.

Although the present embodiment shows patterns in which the lengthwisedirections of lines are arranged in the upward and downward direction,the present invention is not limited thereto. By using patterns in whichlengthwise directions of lines are arranged in a plurality ofdirections, the light transmittance distribution of the light path withrespect to the angular direction of the projection optical system can bemeasured.

FIGS. 8A and 8B are views showing principles on which an inspection iscarried out with use of a pattern in which the lengthwise directions oflines are oriented in a plurality of directions. As shown in FIG. 8A,pinhole patterns 71 a to 71 d having one same diameter of 80 μm areformed on one photomask 70. A diffraction pattern is formed in each ofthese pinhole patterns 71 a to 71 d. Note that the other area than thepatterns 71 a to 71 d is a opaque area 73. FIG. 8A shows enlarged viewsof the diffraction patterns 72 a to 72 d in the patterns 71 a to 71 d.As can be seen from the enlarged views of the diffraction patterns 72 ato 72 d, the diffraction patterns 72 a to 72 d are line and spacepatterns each constructed by opaque parts 74 and light transmissionparts 75, and have one same duty ratio at an equal period. Thediffraction patterns 72 a to 72 d are different from each other inlengthwise directions of their lengthwise directions of the line andspace patterns.

FIG. 8B schematically shows photoresist patterns formed by the photomask70 shown in FIG. 8A. The patterns to be transferred by the patterns 71 ato 71 d respectively correspond to the patterns 76 a to 76 d. As can beseen from FIG. 8B, a zeroth-order diffraction light pattern 32 andpositive and negative first-order diffraction light patterns 33 a and 33b are formed in the light reach area 31. The positive and negativefirst-order diffraction light patterns 33 a and 33 b are formed atpositions shifted in the direction vertical to the lengthwise directionof lines of the diffraction patterns. Accordingly, it is understood thatthe positions of the formed patterns vary by changing the lengthwisedirection of lines of the diffraction patterns.

By thus using a photomask where diffraction gratings are formed in aplurality of directions, it is possible to measure the transmittance ofthe projection optical system corresponding to the plurality ofdirections from the center.

The example shown in FIGS. 8A and 8B has been explained with referenceto patterns whose lengthwise directions are oriented in four directions.Needless to say, however, the present invention is not limited thereto.For example, the distribution of the light transmittance of the lensforming part of the projection optical system can be inspected withrespect to all the directions viewed from the center if lengthwisedirections of the lines of the diffraction patterns are oriented in thedirections so as to cover all the peripheral part of the light reacharea 31 by the first-order diffraction light. For example, in case aphotomask where eight kinds of diffraction patterns are formed in eightdirection at respective angles (22.5°×N) (N: 0, 1, 2, . . . , 7) withrespect to a reference straight line, measurement point is as two timesas in the case of FIGS. 8A and 8B, thereby more precise measurement canbe performed than in the case of FIGS. 8A and 8B. In addition, it isunnecessary to arrange these plural periodic patterns on one photomask,like in the case of the modification shown in FIGS. 6A to 6H.

(Second Embodiment)

FIGS. 9A to 11 are explanatory views for explaining an inspection methodfor an exposure apparatus according to the second embodiment of thepresent invention. The present embodiment will be explained also withreference to an example of the case where an inspection is carried outwith respect to a KrF excimer reduction projection apparatus (λ: 248 nm,NA: 0.6, σ: 0.3, and magnification M of the mask: 4), like the firstembodiment. Detailed explanation of the structure of the exposureapparatus will therefore be omitted. Characteristic points of thepresent embodiment exist in the shape of the diffraction grating in thepinhole which is drawn on the photomask used for inspection. Althoughline and space patterns are arranged in the pinhole patterns in thefirst embodiment, square grating patterns are arranged in the presentembodiment.

FIG. 9A is a view showing the entire structure of the photomask 80 usedin the present embodiment. Note that the present embodiment can beapplied to the exposure apparatus shown in FIG. 2 by providing thephotomask 80 in place of the photomask 3 used in the first embodiment.As shown in FIG. 9A, in the photomask 80 having a size of 15 cm×15 cm,pinhole patterns 81 are arranged at an equal interval. The diameter ofeach pinhole pattern 81 is 80 μm like in the first embodiment, and thepinhole patterns 81 are arranged such that the distance from eachpinhole pattern 81 to another nearest pinhole pattern 81 is 1200 μm.

The diameter of each pinhole pattern 81 is set to 80 μm and the distancebetween pinhole patterns 81 is set to 1200 μm from the followingreasons.

Each pinhole pattern 81 in the present embodiment is based on the sameprinciples as those basing a pinhole camera. It is well known that theresolution of a pinhole camera is optimized when r=(1λ)^(1/2) issatisfied where the radius of the pinhole is r, the light path lengthfrom the pinhole to the surface where an image is projected is 1, andthe wavelength of light is λ.

In case of the present embodiment, pattern images to be transferred to awafer 7 need only to be at non-conjugate positions with respect to theprojection optical system, i.e., the resolution needs to be high in thesurface side of the mask. Accordingly, an image with high resolution canbe obtained by obtaining a product of the thickness d of the photomask 3and a refractive index n of glass forming part of the body of thephotomask 3. In the present embodiment, however, the resolution needsnot always be the highest but needs only to satisfy the followingexpression (1).

0.4(ndλ)^(1/2) ≦r≦(ndλ)^(1/2)  (1)

If a pattern having an area equal to a circle having a radius r, i.e.,πr² is considered other than a circular pattern, it is only necessary tosatisfy the following expression (2).

0.4(ndλ)^(1/2) ≦r≦10(ndλ)^(1/2)  (2)

In the case of the present embodiment, n=1.5, d=6.35 mm, and λ=248 nmare satisfied, so it is understood that the diameter of the pinholepattern 81 satisfies the above expression (1).

If exposure is carried out under the above-described condition, thediameter of the diffraction light reach area 91 in FIG. 10 is about 300μm. That is, the positions where individual patterns are transferredmust be distant from each other at least by 300 nm long on the wafer inorder that images of individual pinholes do not overlap each other. Inthe present embodiment, the magnification M of the mask=4 is given sothat the interval distance between pinhole patterns on the photomaskmust be 1200 μm or more. The pinhole pattern layout on the photomask 80satisfies this condition.

FIG. 9B is an enlarged view showing the vicinity of a pinhole pattern 81arranged on the photomask 80. FIG. 9C is a more enlarged view showing apart of the pattern illustrated in FIG. 9B. As shown in FIGS. 9B and 9C,the pinhole pattern 81 is comprised of a opaque part 82 and lighttransmission parts 83.

The opaque part 82 extends linearly in the longitudinal and lateraldirections of the figure such that the light transmission parts 83disposed in a matrix are arranged to be apart from each other,constructing a grating-like shape. In this manner, the lighttransmission parts function as a diffraction grating with respect to twodirections of X- and Y-directions. Where k is a line width of the opaquepart 82 and a is the length of the side of each light transmission part83, k=0.8 μm and a=0.8 μm are given and the pattern period is 1.6 μm.

Thus, a plurality of pinholes patterns 81 shown in FIGS. 9B and 9C areprovided, and it is therefore possible to observe changes of thetransmittance within the exposure area in the projection optical system.

With use of the photomask 80 described above, pattern exposure iscarried out in a non-conjugate state in which the wafer 5 and thephotomask 80 are not conjugate with respect to the projection opticalsystem. Like the first embodiment, exposure is carried out for aplurality of times with different exposure doses.

FIG. 10 is a view schematically showing photoresist pattern obtained bythe exposure described above. As shown in FIG. 10, the inside of thelight reach area 91 is an area where the light that passes through theprojection optical system 4 can reach the wafer 5. The diffractionpatterns provided on the photomask 80 constitute a grating pattern whichis periodical in two directions. The grating pattern therefore generatesfirst-order diffraction light not only in one direction but also in adirection vertical to the one direction. Accordingly, one zeroth-orderdiffraction pattern 92 and four first-order diffraction light patterns93 to 96 are generated on the wafer 5. These diffraction light patterns92 to 96 are similar to a cross-sectional shape of light emitted fromthe illumination optical system 1 of the exposure apparatus. Inaddition, images that reflect respective diffraction lights are formedon the wafer 5. The first-order diffraction lights 93 to 96 arepartially shielded by the aperture stop in the periphery of the pupil 4a of the projection optical system 4 to have a shape chipped thereby.

This kind of photoresist pattern is observed, for example, by an opticalmicroscope, and boundaries are defined between areas where photoresistwas stripped and areas where photoresist was remained. Specifically,boundaries between the diffraction light patterns 92 to 96 and the otherarea are the boundaries defined in this case.

Pattern exposure described above is carried out for a plurality of timeswhile changing the exposure dose, and as a result, photoresist patternshaving different forming areas can be obtained. FIG. 11 is a plan viewshowing photoresist patterns obtained by total six times of exposure. Asshown in FIG. 11, a photoresist pattern 101 corresponds to a case whereminimum exposure dose can be given, a photoresist pattern 102 can beobtained by an exposure with larger exposure dose than in the case ofthe photoresist pattern 101, a photoresist pattern 103 can be obtainedby an exposure with larger exposure dose than in the case of thephotoresist pattern 102, . . . , and a photoresist pattern 106 can beobtained by an exposure with largest exposure dose. The sizes of thefirst-order diffraction light patterns 93 to 96 vary by changing theexposure dose. It can be understood that the first-order diffractionlight patterns 93 to 96 that can not substantially observed at a lowexposure dose can be observed clearly by increasing the exposure dose.Thus, the boundaries of the diffraction light patterns 93 to 96 vary asthe exposure dose varies.

Based on the six kinds of photoresist patterns 101 to 106 obtained bychanging the exposure dose, the intensity ratio of each of thefirst-order diffraction lights 93 to 96 is obtained. Note that theprocess for obtaining the intensity ratio is the same as that of thefirst embodiment and detailed explanation thereof will be omitted.

Thus, according to the present embodiment, an inspection is carried outwith use of a photomask on which a grating-like diffraction pattern isprovided. In this manner, the same advantages as those of the firstembodiment can be achieved, and information concerning the projectionoptical system can be obtained with respect to one direction and anotherdirection vertical to the one direction.

(Third Embodiment)

FIGS. 12A, 12B and 13 are explanatory views for an inspection method foran exposure apparatus according to the third embodiment of the presentinvention. The present embodiment will be explained with reference to acase of making an inspection of a KrF excimer laser reduction projectionexposure apparatus (λ: 248 nm, NA: 0.6, σ: 0.3, magnification M of themask: 4) like the first embodiment. Therefore, detailed explanation ofthe structure of the exposure apparatus will be omitted herefrom. Thepresent embodiment is characterized in the shape of the diffractiongrating in each pinhole drawn on the photomask used for the inspection.In the first and second embodiments, line and space patterns or squaregrating-like patterns are arranged in each pinhole pattern. In thepresent embodiment, however, a honeycomb-like pattern is arranged.

FIG. 12A is a view showing a main part of the photomask used in thepresent embodiment. As shown in FIG. 12A, pinhole patterns 112 eachhaving a diameter of 80 μm are arranged, surrounded by a opaque area111. Although the present embodiment does not show the entire structureof the photomask, either a case where only one single pinhole pattern112 are formed or a plurality of pinhole patterns are formed can beapplied to the present embodiment.

FIG. 12B is an enlarged view showing the inside of a pinhole pattern 112shown in FIG. 12A. The pinhole pattern 112 is constructed by a opaquepart 113 and circular light transmission parts 114.

A large number of light transmission parts 114 are formed in the holepattern 111 and are arranged such that the light transmission parts 114have periodic relationships in three directions between each other. Thatis, the light transmission parts 114 are arranged at an equal intervalin one direction 115, at an equal interval in a direction at 60° to thedirection 115, and at an equal interval in a direction at 120° to thedirection 115. A light passes through the light transmission parts 114arranged adjacent to each other at equal intervals in the directions 115to 117, and a diffraction occurs due to lights which have passed throughthe light transmission parts 114 arranged adjacent to each other.

Note that regular hexagonal boundaries separating the light transmissionparts 114 each other are drawn merely to facilitate the description, soall of the other area including these boundaries than the lighttransmission parts 114 is the opaque part 113.

Using a photomask where pinhole patterns 112 shown above are arranged,an inspection is carried out like the first and second embodiments.Specifically, exposure is carried out for a plurality of times atdifferent exposure doses. FIG. 13 shows an example of photoresistpatterns formed on the wafer 5 in this manner. As shown in FIG. 13, theinside of the light reach area 121 is an area where the light that haspassed through the photomask and the projection optical system 4 fromthe illumination optical system 1 can reach the wafer. A honeycomb-likepattern provided on the photomask according to the present embodimenthas periods in three directions, and therefore, diffraction phenomenaoccur in six directions. Accordingly, one zeroth-order diffraction lightpattern 122 and six first-order diffraction light patterns 123 to 128are generated.

The photoresist patterns of this kind are observed, for example, by anoptical microscope. It is possible to obtain a changes of thetransmittance of the projection optical system depending on the path oflight of the projection optical system by obtaining a ratio of the lightintensity with use of the same manner as that of the first or secondembodiment, based on the result of this observation, like the first orsecond embodiment.

Thus, according to the present embodiment, the same advantages as thoseof the first and second embodiments are obtained and an inspection iscarried out with use of patterns having periods in three directions, sofirst-order diffraction light patterns can be obtained in six positions.Accordingly, the light transmittance distribution of the light path withrespect to the angular direction of the projection optical system can bemeasured in a shorter time than in the first and second embodiments.

(Fourth Embodiment)

FIGS. 14 to 16 are explanatory views for an inspection method for anexposure apparatus, according to the fourth embodiment of the presentinvention. In the present embodiment, explanation will be made withreference to a case of an example in which a KrF excimer laser reductionprojection exposure apparatus (λ: 248 nm, NA: 0.6, σ: 0.3, maskmagnification M: 4) is inspected. Therefore, detailed explanation of thestructure of the exposure apparatus will be omitted herefrom. Thepresent embodiment is characterized in that a phase shift mask is usedto make an inspection.

FIG. 14 is a view showing the entire structure of a photomask 131 usedin the present embodiment. Note that the present embodiment can beapplied to the exposure apparatus shown in FIG. 2 by providing thephotomask 131 in place of the photomask 3 used in the first embodiment.AS shown in FIG. 14, a pinhole pattern 132 having a diameter of 80 μm isprovided on the photomask 131, and the other area than the pinholepattern 132 is a opaque part 133. Note that only one opaque part 133 isillustrated in FIG. 14 for simplicity, a plurality of opaque parts 133can be arranged like in FIG. 9A, so that each opaque part 133 is formedat least to the position range of 1200 μm from the pinhole pattern 132.

The inside of the pinhole pattern 132 is comprised of a attenuated phaseshift part 134 and a light transmission part 135. The light transmissionpart 135 is comprised of a plurality of square patterns which arearranged like a grating. The ratio between the pattern width of theattenuated phase shift part 134 and the pattern width of the lighttransmission part 135 is 11:9, namely the duty ratio of the diffractiongrating is 045, and the pattern period is 3.2 μm. The lighttransmittance of the attenuated phase shift part 134 is 24.5% and aphase difference between a light which pass through the lighttransmission part 135 and a light which pass through the attenuatedphase shift part 134 is 180°.

With use of the photomask 131 described above, pattern exposure iscarried out in a non-conjugate state in which the wafer 5 and thephotomask 131 are not conjugate with respect to the projection opticalsystem, like the first embodiment. Specifically, exposure is carried outwith pattern-forming surface attached to the mask stage of the exposureapparatus and reversed from that in the case of actual device patternexposure. In addition, like the first embodiment, exposure is carriedout for a plurality of times at different exposure doses.

FIG. 15A is a view schematically showing photoresist patterns obtainedby the exposure described above. The inside of the light reach area 141is an area where the light that passes through the photomask 131 and theprojection optical system 4 from the illumination optical system 1 canreach the wafer 5. The diffraction patterns provided on the photomask131 constitute a grating pattern which is periodical in two directions.The grating pattern therefore generates first-order diffraction lightnot only in one direction but also in another direction vertical to theone direction. Accordingly, four first-order diffraction light patterns142 to 145 are created on the wafer 5. These diffraction light patterns142 to 145 are similar to a cross-sectional shape of light emitted fromthe illumination optical system 1 of the exposure apparatus.

The light intensity ratio between zeroth-order diffraction light and thefirst-order diffraction light which is generated with the diffractiongrating pattern illustrated in the FIG. 13 is expressed by the followingexpression:

S0:S1=[−y+x ²(1+y)]² :[x(1+y)sin(πx)/π]²

Where s0 is the light intensity of the zeroth-order diffraction light,s1 is the light intensity of the first-order diffraction light, x is aduty ratio, and y is an amplitude transmittance. In the case of theattenuated phase shift mask used in the present embodiment, x is 0.45and y is 0.245. Thus, the light intensity of the zeroth-orderdiffraction light is less than {fraction (1/500)} of the light intensityof the first-order diffraction light. Therefore, the intensity ofgenerated zeroth-order first-order diffraction light is very weak sothat the pattern due to zeroth-order diffraction light is nottransferred to the wafer.

This kind of photoresist pattern is observed, for example, by an opticalmicroscope, and boundaries are defined where photoresist was strippedand areas where photoresist was remained. Specifically, boundariesbetween the diffraction light patterns 142 to 145 and the other area arethe boundaries defined in this case.

Pattern exposure described above is carried out for a plurality of timeswhile changing the exposure dose, and as a result, photoresist patternshaving different forming areas can be obtained. Light intensity ratiosbetween first-order diffraction light 142 to 145 can be obtained basedon these photoresist patterns. Note that the process for obtaining theintensity ratio is the same as that of the first embodiment, andtherefore detailed explanation thereof will be omitted herefrom.

Advantages obtained by making an inspection with use of a attenuatedphase shift mask as described above will be explained with reference toFIG. 16. FIG. 16 is a view schematically showing the paths of thefirst-order diffraction light in the present embodiment. In case wherethe photomask has the structure as described above, the intensity of thezeroth-order diffraction light which runs straight is too weak to exposethe photoresist on the wafer 5. Note that zeroth-order light is omittedfrom FIG. 16. Primary diffraction lights are generated in fourdirections at predetermined angles, and reach the wafer 5 through pathsdifferent from each other, respectively, thereby exposing thephotoresist.

In the present embodiment, no zeroth-order light is generated, and it istherefore possible to measure components of the first-order diffractionlight that are difficult to obtain dye to existence of zeroth-orderdiffraction light in the first to third embodiment. For comparison, FIG.15B schematically shows photoresist patterns in case of using a normalmask not formed of a attenuated phase shift mask but formed of only alight transmission part and opaque parts. As shown in FIG. 15B, areaswhere the first-order diffraction light patterns 142 to 145 are formedoverlap an area where the zeroth-order diffraction pattern 146 is formedon the wafer 5. These overlapping areas are areas that are formed bylight which passes through a part close to the center axis of theoptical system. Thus, in the parts where the zeroth-order diffractionlight and the first-order diffraction light overlap each other, it isimpossible to observe only the first-order diffraction light patterns142 to 145. Accordingly, it is not possible to obtain informationconcerning the light transmittance of the projection optical system 4.In contrast, if a attenuated phase shift mask is used as in the presentembodiment, the zeroth-order diffraction light becomes a very weakcomponent so that no zeroth-order diffraction light 146 is formed.Accordingly, an overlapping area as shown in FIG. 15B is not generatedbut the light transmittance of the path through which the first-orderdiffraction light which reaches an area close to the center of theprojection optical system 4 can be measured.

Of course, the shape of each pinhole pattern 132, the shape of thediffraction pattern in each pinhole pattern 132, the directions thereof,and the σ value thereof and the like are not limited to values describedabove. Any photomask can be used as long as it is a attenuated phaseshift mask and generates diffraction light. In addition, the phasedifference between the attenuated phase shift part 134 and the opaquepart 135 needs not always be 180° but may be 120° or so. In this case,it is desirable to adjust the phase difference and the same measurementas well as the above measurement can be done.

The present invention is not limited to the embodiments described above.The embodiments described above show only the case where measurement iscarried out with use of first-order diffraction light, but can becarried out with use of second-order diffraction light or higher. Inaddition, zeroth-order diffraction light can be used for measurement. Inthis case, light intensities of zeroth-order diffraction light andfirst-order diffraction light are previously calculated, andzeroth-order diffraction light and first-order diffraction lightactually obtained by transferring patterns may be compared with eachother, based on the light intensities.

In the modification shown in FIGS. 8A and 8B according to the firstembodiment, diffraction patterns having different line lengthwisedirections are arranged individually as four pinhole patterns 71 a to 71d. However, it is possible to arrange a diffraction pattern in which onepinhole pattern includes a plurality of line lengthwise directions.FIGS. 17A and 17B are views showing this modification example. In themask pattern 161 shown in FIG. 17A, diffraction patterns 161 a and 161 bhaving two kinds of line lengthwise directions are arranged in a opaquearea 161 c. In the mask pattern 162 shown in FIG. 17B, diffractionpatterns 162 a to 162 d having four kinds of line lengthwise directionsare arranged in a opaque area 162 e. In case of photoresist patternstransferred by the mask pattern 161 shown in FIG. 17A, a zeroth-orderdiffraction light pattern 32 and four first-order diffraction lightpatterns 33 a to 33 d are formed. In case of the photoresist patternstransferred by the mask pattern 132 shown in FIG. 17B, a zeroth-orderdiffraction light pattern 32 and eight first-order diffraction lightpatterns 33 a to 33 h are formed.

By thus making an inspection with a diffraction pattern having aplurality of line lengthwise directions in one pinhole pattern, the sameadvantages as those of the modification example shown in FIGS. 8A and 8Bcan be obtained and more remarkable advantages can further be obtainedthan them. That is, in the case of the modification example shown inFIGS. 8A and 8B, information concerning eight directions from the centerof a projection optical system is obtained by synthesizing four obtainedphotoresist patterns, with respect to a projection optical system. Inthe case of the example shown in FIGS. 17A and 17B, informationconcerning a plurality of directions can be obtained from onephotoresist pattern without synthesizing photoresist patterns.Accordingly, inspections can be made without being influenced fromerrors between respective images when synthesizing images obtained fromphotoresist patterns. It is therefore possible to make an inspectionwith high precision.

Alternatively, an alternating phase shift mask may be used. For example,in the diffraction patterns shown in FIG. 3B in the embodiment describedabove, lights which pass through light transmission parts 26 adjacent toeach other may be arranged to have a phase difference of 180° betweeneach other. In this case, the duty ratio may be any value. Although theexplanation has been made with reference to FIG. 3B, the phasedifference between lights which pass through the closest lighttransmission parts needs only to be set to 180°. For example, in case ofFIG. 9B, it is only necessary that light which passes through each oflight transmission parts 83 positioned to a light transmission part 83as a reference in the longitudinal and lateral directions from have aphase difference of 180° with respect to light which passes through thelight transmission part 83 as the reference.

Although circular pinhole patterns are obtained as patterns arranged ona photomask, the present invention is not limited thereto. FIGS. 18A to18C show modification examples of the photomask. Any pinhole patternscan be used as long as the periphery of each hole pattern is shieldedfor a predetermined distance by a opaque part, like a pattern in whichtriangular hole patterns 172 are formed in the photomask 171 a, as shownin FIG. 18A, a pattern in which rectangular hole patterns 173 are formedin a photomask 171 b, as shown in FIG. 18B, and a pattern in whichelliptic hole patterns 174 are formed in a photomask 171 c, as shown inFIG. 18C. If the hole patterns have a shape whose size is equal to acircular hole pattern, the same diffraction light as that in theembodiments described above is generated and the same photoresistpattern as that in the embodiment is obtained.

In case of thus using triangular, rectangular, or elliptic holepatterns, the condition indicated by the expression (1) in the firstembodiment is normalized as in the following expression (3).

0.4(ndλ)^(1/2) ≦s/2≦(ndλ)^(1/2)  (3)

The s in this expression is a length of the longest line among lengthsof lines connecting arbitrary two points on the boundaries between thelight transmission patterns and the opaque pattern.

Although the above embodiments shows the cases where a line and spacepattern, a square grating pattern, and a honeycomb-like pattern are usedas diffraction gratings arranged inside the pinhole patterns, thepresent invention is not limited thereto. Any diffraction patterns canbe used as long as the patterns generate dispersive diffraction light,for example, like a checkered grating pattern, a pillar pattern in whichopaque parts and light transmission parts of these patterns arereversed, and the like.

FIG. 19 shows an example of a checkered grating pattern. As shown inFIG. 19, a pinhole pattern 192 is provided, surrounded by a opaque area191. The point that a plurality of pinhole patterns of this kind areprovided in an actual mask pattern is the same as FIGS. 8A, 9A, 12A, andthe like.

As can be seen from an enlarged view of this pinhole pattern in FIG. 19,this pinhole pattern is constructed by a opaque part 194 and lighttransmission parts 193. Each light transmission part 193 is surroundedby the opaque part 193 and is a square. Suppose that one of the squarelight transmission parts 193 is used as a reference and is denoted by193 a. Light-transmissible parts 193 b, 193 c, 193 d, and 193 e havingtheir gravity centers on diagonals 195 a and 195 b are provided atpositions closest to the light transmission part 193 a. If each lighttransmission part is used as a reference, light transmission parts areprovided at closest positions under the same condition as those of thelight transmission part 193 a. FIG. 20 shows an example of a resistpattern which is transferred to a wafer by the checkered-grating pinholepattern constructed as described above. As shown in FIG. 20, azeroth-order diffraction light pattern 202 is formed by zeroth-orderdiffraction light, and first-order diffraction light patterns 203 a to203 d are formed by first-order diffraction lights, within a right reacharea 201. By thus using a checkered grating pattern, first-orderdiffraction light patterns can be formed in four directions. Further,first-order diffraction light patterns can be formed in the diagonaldirections since closest light transmission parts are adjacent to eachother in the diagonal directions (45° and 135° with respect to thelongitudinal and lateral directions of each light transmission part193). This is a significant advantage, considering that a maskmanufactured by a drawing device using an electron beam or light isnormally created by drawing rectangular pixels as units. That is, it isnecessary to manufacture a mask pattern in which longitudinal andlateral directions of a rectangle are inclined obliquely, in order toform first-order diffraction light patterns in the oblique directionswithout using a checkered grating pattern as described above. However,manufacture of a mask pattern of this kind is relatively difficult dueto drawing techniques.

Further, in the embodiments described above, a plurality of transferpatterns obtained by changing the exposure dose are subjected to imageprocessing, and images are synthesized, thereby to obtain a contour plotof intensities. However, the present invention is not limited thereto.For example, a light intensity distribution may be obtained from thefilm thickness of photoresist pattern obtained by inspection exposure,and intensity distributions of respective patterns thus obtained may besynthesized, thereby to obtain a contour plot of light intensitydistributions. Based on the contour plot of light intensitydistributions, the light transmittance of the projection optical systemcan be obtained. In the case of this method, the light transmittance canbe obtained by preparing and referring to a sensitivity curve expressingthe relationship between the film thickness of the photoresist which hasexposed and the light intensity, without using a resist having noproportional relationship. It is preferable to use photoresist in whichthe light intensity and the film thickness of photoresist patternsconstitute a proportional relationship or a similar relationship. Ofcourse, a light transmittance can be obtained by the exposure withappropriate exposure doses and referring to a sensitivity curveexpressing the relationship between the film thickness and the lightintensity, without using a resist having no proportional relationship.For example, if a sensitivity curve is expressed such as in FIG. 22, itis preferable to be exposed with exposure doses with respect to a range204 of the intensity in which the slope of the sensitivity curve isrelatively larger than another range.

This measurement method will now be explained with a resist patternshown in FIG. 4 taken as an example.

With a resist pattern of this kind, a film thickness distribution ismeasured, for example, by a film thickness measuring device. Changes ofthe film thickness are obtained throughout the entire area wherediffraction patterns of positive and negative first-order diffractionlights are transferred. Specifically, the changes can be expressed inform of a contour map.

In general, the sensitivity and decomposition characteristics ofphotoresist with respect to light differ depending on the kinds ofphotoresist. For example, a sensitivity curve as shown in FIG. 21, whichshows the relationship between the light intensity and the thickness ofa residual film, is obtained. Further, the contour map of the filmthickness distribution as described above is substituted into adistribution graph of intensity of light irradiated on a wafer, byreferring to the sensitivity curve. From a light intensity distributiongraph thus obtained, a difference between positive and negativefirst-order diffraction lights is obtained.

Although explanation has been made with use of FIG. 4 in this example,the same is also applicable to other resist patterns.

Although a KrF excimer laser light source is used as an inspectiontarget of the exposure apparatus, the same advantages as those of thepresent invention can be obtained even in case where, for example, ani-ray or ArF excimer laser, F₂ excimer laser, or the like is used as alight source. In addition, the mask magnification M and NA are notlimited to values shown in the above embodiments. Although the photomask3 is explained as a mask for inspection of an exposure apparatus, anactual pattern that is used for actual pattern exposure may be arrangedwith its surfaces reversed from those of a pinhole pattern used forinspection. In this manner, it is possible to carry out actual patternexposure and to observe simply the light transmittance of the projectionoptical system on real time, using a pattern for inspection for everytime of the actual exposure. In this case, the pinhole pattern and theactual pattern should preferably be arranged apart from each other to anextent that both patterns do not interfere with each other.

As has been explained above, according to the present invention, it ispossible to specify changes of the light transmittance of the projectionoptical system depending on the paths of lights.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method for inspecting an exposure apparatus,comprising: a step of guiding light emitted from an illumination opticalsystem to a photomask where a pattern is formed of an optical memberincluding a light transmission pattern as a diffraction grating pattern,in which a light transmission part and an opaque part are repeated in apredetermined direction, a plurality of ratios are given between alength of the light transmission part and a length of the opaque part ina repetition direction, and a periphery of the light transmissionpattern is shielded by an opaque area, such that a plurality of ratiosare given between the light transmission part and the opaque part; astep of irradiating diffraction light, which has passed through thephotomask, on a projection optical system, thereby to transfer a patternreflecting an intensity distribution of the diffraction light to awafer; and a step of measuring a change of transmittance depending on alight path of the projection optical system, based on a pattern image ofthe diffraction light transferred to the wafer.
 2. A method according toclaim 1, wherein said pattern transfer is performed in which thephotomask and the wafer are non-conjugate with respect to the projectionoptical system.
 3. A method according to claim 1, wherein, where NA is anumerical aperture of the projection optical system in a side of thewafer, λ is a exposure length, σ is a coherence factor, and M is amagnification of the photomask, the diffraction grating pattern has aperiod which satisfies p>Mλ/NA(1+σ).
 4. A method according to claim 1,wherein the non-conjugate state in which the photomask and the wafer arenon-conjugate with respect to the projection optical system is realizedby arranging the opaque part of the light optical member on a surfaceopposite to a surface where the optical member of the photomask used fordevice pattern exposure is arranged.
 5. A method according to claim 1,wherein, where a length of a longest line among lengths of linesconnecting arbitrary two points positioned on a boundary to the opaquepart, of the opaque part of the light transmission pattern, is 2r, athickness of the photomask is d, an exposure wavelength is λ, and arefractive index of a material of the photomask at the exposurewavelength λ is n, a relationship of 0.4(ndλ)^(1/2)≦r≦(ndλ)^(1/2) issatisfied.
 6. A method according to claim 5, wherein the lighttransmission pattern is a circular pattern having a radius r.
 7. Amethod according to claim 6, wherein where, of the light pattern, anarea surrounded by the opaque area is expressed λr², a thickness of thephotomask is d, an exposure wavelength is λ, and a material of thephotomask has a refractive index of n, a relationship of0.4(ndλ)^(1/2)≦r≦10(ndλ)^(1/2) is satisfied.
 8. A method according toclaim 1, wherein the pattern formed on the wafer is made of apredetermined material, and the change of the transmittance is measuredby measuring a film thickness of the pattern transferred to the waferand by obtaining a light intensity of the diffraction light, based on apredetermined relationship between a film thickness of the predeterminedmaterial and an irradiation light intensity.
 9. A method according toclaim 1, wherein the predetermined relationship between the filmthickness of the predetermined material and the light intensity is asensitivity curve expressing the relationship between the film thicknessof the predetermined material and the light intensity.
 10. A methodaccording to claim 1, wherein a change of the transmittance is measuredin a manner that a boundary between an area where photoresist wasstripped and an area where photoresist was remained is regarded as aequal-intensity contour curve, a plurality of equal-intensity contourcurves each being the equal-intensity contour curve are obtainedrespectively under different conditions, and the plurality ofequal-intensity contour curves obtained are layered thereby to obtain anequal-intensity contour plot.
 11. A method for inspecting an exposuredevice, comprising: a step of guiding light emitted from an illuminationoptical system to a photomask where a pattern is formed of an opticalmember including a light transmission pattern as a diffraction gratingpattern, in which a light transmission part and an opaque part arerepeated in a predetermined direction, a plurality of ratios are givenbetween a length of the light transmission part and a length of theopaque part in a repetition direction, phases of lights which passthrough adjacent light transmission parts with the opaque part insertedtherebetween differs from each other substantially by 180°, and aperiphery of the light transmission pattern is shielded by an opaquearea, such that a plurality of ratios are given between the lighttransmission part and the opaque part; a step of irradiating diffractionlight, which has passed through the photomask, onto a projection opticalsystem, thereby to transfer the pattern to a wafer and to form a patternreflecting an intensity distribution of the diffraction light; and astep of measuring a change of transmittance depending on a light path ofthe projection optical system, based on a pattern image of thediffraction light transferred to the wafer.
 12. A method according toclaim 11, wherein said pattern transfer is performed in which thephotomask and the wafer are non-conjugate with respect to the projectionoptical system.
 13. A method according to claim 11, wherein the patternformed on the wafer is made of a predetermined material, and the changeof the transmittance is measured by measuring a film thickness of thepattern transferred to the wafer and by obtaining a light intensity ofthe diffraction light, based on a predetermined relationship between afilm thickness of the predetermined material and an irradiation lightintensity.
 14. A method according to claim 11, wherein the predeterminedrelationship between the film thickness of the predetermined materialand the light intensity is a sensitivity curve expressing therelationship between the film thickness of the predetermined materialand the light intensity.
 15. A method according to claim 11, wherein achange of the transmittance is measured in a manner that a boundarybetween an area where photoresist was stripped and an area wherephotoresist was remained is regarded as a equal-intensity contour curve,a plurality of equal-intensity contour curves each being theequal-intensity contour curve are obtained respectively under differentconditions, and the plurality of equal-intensity contour curves obtainedare layered thereby to obtain an equal-intensity contour plot.